1. Field of the Invention
The present invention relates to a method of producing an energy trapped type piezoelectric resonator vibrating in the thickness expansion mode, and more particularly, it relates to a method of producing a piezoelectric resonator which is provided with a cavity inside an armoring resin member containing vibrating electrode parts thereof.
2. Description of the Background Art
In an energy trapped type piezoelectric resonator vibrating in the thickness expansion mode, main vibration is inevitably damped if armoring resin adheres to its vibrating electrode parts. Therefore, it is necessary to define a cavity around the vibrating electrode parts. Japanese Patent Publication Gazette No. 45-22384 (1970) discloses a conventional method of preliminarily applying a cavity forming material such as wax or paraffin, which is solid or semi-solid at room temperatures, easily softened upon heating and melting at a relatively low temperature, onto vibration electrode parts and heating it in or after sealing with armoring resin, for melting the cavity forming material and causing it to be absorbed by the armoring resin.
In this method, however, the cavity forming material is applied first onto a vibrating electrode part which is provided on a front surface of a piezoelectric substrate and then the substrate is turned over so that to apply the cavity forming material may be applied onto another vibrating electrode part which is provided on its back surface. Since the cavity forming material is applied onto the vibrating electrode parts in two steps, workability is poor. In case of a composite part comprising a resonator and a different element, such as a capacitor, arranged in parallel relationship with each other and the direction of thickness as described in Japanese Utility Model Laying-Open Gazette No. 1-74626 (1989), for example, it is difficult by this method to apply the cavity forming material onto a vibrating electrode part of the resonator.
In order to solve the aforementioned problem, Japanese Patent Laying-Open Gazette No. 1-125011 (1989) proposes a method of applying a cavity forming material onto vibrating electrode parts provided on front and back surfaces of a piezoelectric substrate in a single step without turning over the substrate. According to this method, the piezoelectric substrate is vertically arranged and the cavity forming material is supplied from above its upper edge and is simultaneously applied onto vibrating electrode parts on front and back surfaces of the piezoelectric substrate.
This method would be workable if the cavity forming the material were to be applied only over the vibrating electrode parts. In practice, however, the cavity forming material remains on the upper edge of the piezoelectric substrate, thereby causing an unwanted cavity to be created therealong when the periphery of the piezoelectric substrate is sealed with armoring resin. Although a piezoelectric resonator generally develops a response in an odd higher mode, it is possible to effectively suppress higher mode spurious vibrations without disturbing main responses if the cavity is created only around the vibrating electrode parts and is tightly sealed the same with armoring resin. However, if an unwanted cavity is created along the upper edge of the piezoelectric substrate as described above, energy that can be trapped is not sufficient and this is likely to cause higher mode spurious vibrations.